The head of decommissioning for the damaged Fukushima nuclear plant said Thursday that more creativity is needed in developing robots to locate and assess the condition of melted fuel rods.

A robot sent inside the No. 2 reactor containment vessel last month could not reach as close to the core area as was hoped for because it was blocked by deposits, believed to be a mixture of melted fuel and broken pieces of structures inside. Naohiro Masuda, president of Fukushima Daiichi Decommissioning, said he wants another probe sent in before deciding on methods to remove the reactor’s debris.

The No. 2 reactor is one of the Fukushima reactors that melted down following the 2011 earthquake and tsunami.

The plant’s operator, Tokyo Electric Power Co (TEPCO), needs to know the melted fuel’s exact location as well as structural damage in each of the three wrecked reactors to figure out the best and safest ways to remove the fuel. Probes must rely on remote-controlled robots because radiation levels are too high for humans to survive.

Despite the incomplete probe missions, officials have said they want to stick to their schedule to determine the removal methods this summer and start work in 2021.

Earlier probes have suggested worse-than-anticipated challenges for the plant’s cleanup, which is expected to take decades. During the No. 2 reactor probe in early February, the “scorpion” robot crawler stalled after its total radiation exposure reached its limit in two hours, one-fifth of what was anticipated.